Negative-strand RNA viruses are a group of animal viruses that comprise several important human pathogens, including influenza, measles, mumps, rabies, respiratory syncytial, Ebola and hantaviruses.
The genomes of these RNA viruses can be unimolecular or segmented, and are single stranded of (−) polarity. Two essential requirements are shared between these viruses: their genomic RNAs must be efficiently copied into viral RNA, a form which can be used for incorporation into progeny virus particles and transcribed into mRNA which is translated into viral proteins. Eukaryotic host cells typically do not contain the machinery for replicating RNA templates or for translating polypeptides from a negative-stranded RNA template. Therefore negative-strand RNA viruses encode and carry an RNA-dependent RNA polymerase to catalyze synthesis of new genomic RNA for assembly into progeny viruses and mRNAs for translation into viral proteins.
Genomic viral RNA must be packaged into viral particles in order for the virus to be transmitted. The processes by which progeny viral particles are assembled and the protein/protein interactions that occur during assembly are similar within the RNA viruses. The formation of virus particles ensures the efficient transmission of the RNA genome from one host cell to another within a single host or among different host organisms.
Virus families containing enveloped, single-stranded RNA with a negative-sense genome are classified into groups having non-segmented genomes (Paramyxoviridae, Rhabdoviridae, Filoviridae and Borna Disease Virus, Togaviridae) and those having segmented genomes (Orthomyxoviridae, Bunyaviridae and Arenaviridae). The Orthomyxoviridae family includes the viruses of influenza, types A, B and C viruses, as well as Thogoto and Dhori viruses and infectious salmon anemia virus.
Influenza virions consist of an internal ribonucleoprotein core (a helical nucleocapsid) containing the single-stranded RNA genome, and an outer lipoprotein envelope lined inside by a matrix protein (M1). The segmented genome of influenza A virus consists of eight molecules of linear, negative polarity, single-stranded RNAs which encode eleven polypeptides (ten in some influenza A strains), including: the RNA-dependent RNA polymerase proteins (PB2, PB1 and PA) and nucleoprotein (NP) which form the nucleocapsid; the matrix membrane proteins (M1, M2); two surface glycoproteins which project from the lipid-containing envelope: hemagglutinin (HA) and neuraminidase (NA); the nonstructural protein (NS1) and nuclear export protein (NEP). Most influenza A strains also encode an eleventh protein (PB1-F2) believed to have proapoptotic properties.
Transcription and replication of the viral genome takes place in the nucleus and assembly occurs via budding on the plasma membrane. The viruses can reassort genes during mixed infections. Influenza virus adsorbs via HA to sialyloligo-saccharides in cell membrane glycoproteins and glycolipids. Following endocytosis of the virion, a conformational change in the HA molecule occurs within the cellular endosome which facilitates membrane fusion, thus triggering uncoating. The nucleocapsid migrates to the nucleus where viral mRNA is transcribed. Viral mRNA is transcribed by a unique mechanism in which viral endonuclease cleaves the capped 5′-terminus from cellular heterologous mRNAs which then serve as primers for transcription of viral RNA templates by the viral transcriptase. Transcripts terminate at sites 15 to 22 bases from the ends of their templates, where oligo(U) sequences act as signals for the addition of poly(A) tracts. Of the eight viral RNA molecules so produced, six are monocistronic messages that are translated directly into the proteins representing HA, NA, NP and the viral polymerase proteins, PB2, PB1 and PA. The other two transcripts undergo splicing, each yielding two mRNAs which are translated in different reading frames to produce M1, M2, NS1 and NEP. In other words, the eight viral RNA segments code for eleven proteins: nine structural and 2 non-structural (NS1 and the recently identified PB1-F2) proteins.
The generation of modern vaccines for influenza viruses, especially for highly pathogenic avian influenza viruses, relies on the use of reverse genetics, which allows the production of influenza viruses from DNA. The first reverse genetic systems for construction of negative-strand RNA influenza viruses involved the transfection of a single viral gene mixed with in-vitro reconstituted ribonucleoprotein (RNP) complexes and subsequent infection with an influenza helper virus. RNP complexes were made by incubating synthetic RNA transcripts with purified NP and polymerase proteins (PB1, PB2 and PA) from influenza viruses, and a helper virus was used as an intracellular source of viral proteins and of the other vRNAs (Luytjes et al., 1989, Cell, 59, 1107-1113).
Neumann et al. (1994, Virology, 202, 477-479) achieved RNP formation of viral model RNAs in influenza-infected cells after expression of RNA from a murine RNA polymerase I promoter-responsive plasmid. Pleschka et al. (1996, J. Virol., 4188-4192) described a method wherein RNP complexes were reconstituted from plasmid-based expression vectors. Expression of a viral RNA-like transcript was achieved from a plasmid containing a truncated human polymerase I (polI) promoter and a ribozyme sequence that generated a 3′ end by autocatalytic cleavage. The polI-driven plasmid was cotransfected into human 293 cells with polII-responsive plasmids that expressed the viral PB1, PB2, PA and NP proteins. Transfection efficiency was very low, however, with approximately 10 transfectant virus particles per transfection. Additionally, this plasmid-based strategy was dependent on the aid of a helper virus.
In WO 01/04333, segmented negative-strand RNA viruses were constructed using a set of 12 expression plasmids for expressing genomic vRNA segments and RNP proteins. The vectors described in WO 01/04333 were based on well known pUC19 or pUC18 plasmids. According to the description, this system requires a set of 8 plasmids expressing all 8 segments of influenza virus together with an additional set of 4 plasmids expressing nucleoprotein and subunits of RNA-dependent RNA polymerase (PB1, PB2, PA and NP).
WO 00/60050 covers a set of at least two vectors comprising a promoter operably linked to an influenza virus segment cDNA (PA, PB1, PB2, HA, NP, NA, M) and linked to a transcription termination sequence, and at least two vectors comprising a promoter operably linked to an influenza virus segment DNA (PA, PB1, PB2, NP). This system attempted to overcome the difficulties in using of a large number of different vectors by using plasmids with eight RNA polymerase I transcription cassettes for viral RNA synthesis combined on one plasmid.
WO 01/83794 discloses circular expression plasmids comprising an RNA polymerase I (polI) promoter and a polI termination signal, inserted between a RNA polymerase II (polII) promoter and a polyadenylation signal. The term vector according to this application is described as a plasmid which generally is a self-contained molecule of double-stranded DNA that can accept additional foreign DNA and which can be readily introduced into a suitable host cell.
WO 2009/00891 describes a linear expression construct and its use for expression of influenza virus gene segments.
Ozawa M. et al (J. Virol, 2007, vol. 81, pp. 9556-9559) describes a reverse genetics system for the generation of influenza A virus using adenovirus vectors. Hoffmann E. et al (Virology, 2000, 267, pp. 310-317) disclose a system for creating influenza virus by generating viral RNA and mRNA from one template using a bidirectional transcription construct. The rescue of influenza B virus from eight plasmids was also disclosed in Hoffmann et al. (Proc. Natl. Acad. Sci., 2002, 99, pp. 11411-11416).
Epidemics and pandemics caused by viral diseases are still claiming human lives and are impacting the global economy. Influenza is responsible for millions of lost work days and visits to the doctor, hundreds of thousands of hospitalizations worldwide (Couch 1993, Ann. NY. Acad. Sci 685; 803,), tens of thousands of excess deaths (Collins & Lehmann 1953 Public Health Monographs 213:1; Glezen 1982 Am. J. Public Health 77:712) and billions of Euros in terms of health-care costs (Williams et al. 1988, Ann. Intern. Med. 108:616). When healthy adults get immunized, currently available vaccines prevent clinical disease in 70-90% of cases. This level is reduced to 30-70% in those over the age of 65 and drops still further in those over 65 living in nursing homes (Strategic Perspective 2001: The Antiviral Market. Datamonitor. p. 59). The virus's frequent antigenic changes further contribute to a large death toll because not even annual vaccination can guarantee protection. Hence, the U.S. death toll rose from 16,363 people in 1976/77 to four times as many deaths in 1998/99 (Wall Street Journal, Flu-related deaths in US despite vaccine researches. Jan. 7, 2003).
Especially in case of the outbreak of pandemic viral diseases, it can be of utmost importance to provide vaccinations or treatments immediately after outbreak of the disease. In view of the urgent need for providing efficient protection and treatment of viral diseases there is a still high demand for the development of economic, fast and efficient expression systems for virus production which can overcome the disadvantages and difficulties of the present expression technologies and provide an alternative method for virus expression. The object is achieved by the provision of the embodiments of the present application.